To improve high temperature stability, and to give better physical and electrical properties over amine cured epoxy resin systems, it has been the general practice in the epoxy technological field to use anhydride curing agents with epoxy resins, particularly for high voltage insulation applications. Most epoxy-anhydride formulations require elevated-temperature cures, and for most commercial applications it is necessary to add some material to speed the rate of cure. Consequently, a considerable amount of effort has been devoted in recent years to develop a perfect catalyst or accelerator for curing epoxy resins, especially those used for high voltage coil insulation, i.e., over about 7,000 volts. In high voltage coils, only an absolute minimum of voids can be tolerated in the resinous insulation. Therefore, the applied resin impregnating composition must be extremely fluid, solventless, and capable of a very fast gel, so that resin will not easily drain from a coil
The properties desired of such a catalyst or accelerator are: it should be inexpensive and readily available; it should gel the epoxy resin system at times below 60 minutes, and preferably below 50 minutes, at about 150.degree. C.; it should be completely soluble with the epoxy resin-anhydride system at all temperatures; the initial viscosity of the catalyzed resin system should be below about 350 cps. at 25.degree. C.; the storage life of the catalyzed resin system should be over at least 60 days, and preferably about 120 days at 25.degree. C., i.e., the viscosity should remain below about 1,000 cps. during that period; it should not adversely affect the mechanical properties of the cured resin system; after cure, the resin system should have power factor values of below about 10% at 150.degree. C.
Several latent catalysts have appeared on the commercial scene in recent years. Included are quaternary ammonium halides such as benzyltrimethyl-ammonium chloride, stannous octoate, "extra-coordinate" siliconate salts, triethanolamine borate, triethanolamine titanate and various other metal chelates. However, all of these materials failed to meet all of the above described requirements and have been rejected.
Mlyashiro, in U.S. Pat. No. 3,624,032, taught catalytic cures of solid, particulate, epoxy-anhydride transfer molding compositions, by using 0.1 to 10 parts/100 parts epoxy of a zinc, iron, aluminum, copper, cobalt, nickel, magnesium, zirconium or tin acetylacetonate catalytic hardener.
Markovitz, in U.S. Pat. No. 3,812,214, taught catalytic cures of relatively viscous, epoxy resin systems, having initial viscosities of about 375 cps. to 200 cps. at 25.degree. C., by using up to 20 wt.% of a combination phenolic resin accelerator--metal acetylacetonate catalytic hardener. Markovitz eliminated anhydride curing agents, and substituted phenolic resin accelerators and the use of up to 5 wt.% of metallic acetylacetonates. The useful acetylacetonates could contain essentially any metal anion.
Smith, in U.S. Pat. No. 4,026,862, taught the use of amines, imidazoles and quaternary organic onium salts as latent catalysts, for a solventless, highly fluid, resinous, epoxyanhydride impregnating composition, having a mono-carboxylic acid storage stabilizer. British Pat. No. 1,428,561, taught the use of activated chromium-2-ethyl hexanoate as catalyst, and quaternary phosphonium halides or tertiary amines as co-catalysts to provide rapid room temperature gellation for a liquid epoxy-anhydride potting resin system.
Meyers, in U.S. Pat. No. 3,678,004, taught catalytic cures of fluid, epoxy-solvent, non-anhydride systems, using aluminum acetylacetonate as a catalytic hardener, and mineral inorganic, carboxylic, halogenated carboxylic, hydroxyl substituted carboxylic or aryl sulfonic acids as storage stabilizers.
Epoxy-anhydride systems are still very useful. What is needed is an improved epoxy resin system, which still contains anhydride curing agents, but which will have good pot life, and improved gel times and electrical properties. The epoxy-anhydride system should be very fluid, i.e., initially below about 350 cps. at 25.degree. C., to ensure its usefulness as an impregnating composition for high voltage applications, where complete coil impregnation is critical.